The processing of photosensitive material involves a series of steps such as developing, bleaching, fixing, washing, and drying. These steps lend themselves to mechanization by conveying a continuous web of film or cut sheets of film or photographic paper sequentially through a series of stations or tanks, each one containing a different processing liquid appropriate to the process step at that station.
There are various sizes of photographic film processing apparatus, i.e., large photofinishing apparatus and microlabs. A large photofinishing apparatus utilizes tanks that contain approximately 100 liters of each processing solution. A small photofinishing apparatus or microlab utilizes tanks that may contain less than 10 liters of processing solution
The chemicals contained in the processing solution: cost money to purchase; change in activity and are seasoned by the constituents of the photosensitive material that leach out during the photographic process; and after the chemicals are used the chemicals must be disposed of in an environmentally safe manner. Thus, it is important in all sizes of photofinishing apparatus to reduce the volume of processing solution. The prior art suggest various types of replenishing systems that add or subtract specific chemicals to the processing solution to maintain a consistency of photographic characteristics in the material developed. It is possible to maintain reasonable consistency of photographic characteristics only for a certain period of replenishment. After a processing solution has been used a given number of times, the solution is discarded and a new processing solution is added to the tank.
Activity degradation due to instability of the chemistry, or chemical contamination, after the components of the processing solution are mixed together causes one to discard the processing solution in smaller volume tanks more frequently than larger volume tanks. Some of the steps in the photographic process utilize processing solutions that contain chemicals that are unstable, i.e., they have a short process life. Thus, processing solutions in tanks that contain unstable chemicals are discarded more frequently than processing solutions in tanks that contain stable chemicals.